Image display device

ABSTRACT

An image display device includes an image display element, a light source, an illumination system, a projector system comprising a refractive optical system including a plurality of lens groups, and a mirror train including a curved mirror, a first focus structure configured to move the respective lens groups of the refractive optical system by different amounts along a normal line of the image display element, and a second focus structure configured to move the respective lens groups along the normal line of the image display element by different amounts from those of the first focus structure.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority from JapanesePatent Application No. 2011-242679, filed on Nov. 4, 2011, thedisclosure of which is hereby incorporated by reference in its entirety.

In addition, the present application is related to the U.S. patentapplication entitled “Magnification Optical System”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projector type image display devicewhich uses a reflective light bulb and can be implemented as variouskinds of projector.

2. Description of the Related Art

Referring to FIG. 1, the structure and function of a general projectortype image display device using a reflective light bulb is described.

In FIG. 1 a projector includes a reflective light bulb LB as an imagedisplay element, a light source LS, a mirror M, an integrator rod IR, alens LN, a curved mirror CM, and a projector system POS.

The light source LS includes a lamp LP and a reflector RF to project alight beam to the light bulb LB.

The integrator rod IR, lens LN, mirror M, and curved mirror CMconstitute an illumination system to guide the light beam from the lightsource LS to the light bulb LB.

The integrator rod IR is a light pipe made of four mirrors combined as atunnel, to reflect an incident light beam with mirror surfaces to anexit.

The projector system POS projects the reflected beam from the light bulbLB onto a target surface or a screen to form an enlarged image thereon.The light bulb LB is a digital micro mirror device (DMD) in which micromirrors are arranged in array. The normal line of the micro mirrors canbe changed independently from each other by ±12 degrees, for example.

The light from the lamp LP is reflected by the reflector RF, convergedon the entrance of the integrator rod IR, repeatedly reflected therein,and projected as a light beam with uniform luminance. Then, the lightbeam illuminates the light bulb LB via the illumination system.

The illumination system converts the light beam from the integrator rodIR to a surface light source with uniform luminance and forms an imageof the surface light source on the light bulb LB.

The positions of the light bulb LB and the projector system POS aredetermined so that light is reflected by the micro mirrors in the lightbulb LB to be incident on the projector system POS when the micromirrors are inclined by −12 degrees and light reflected thereby is notincident on the projector system POS when the micro mirrors are inclinedby +12 degrees. Then, the direction in which the light beam from thecurved mirror CM is incident on the light bulb LB is decided.

An image can be displayed on the light bulb LB by adjusting theinclination of each micro mirror in accordance with the pixels of animage projected on a target surface.

By illuminating the light bulb LB on which the image is displayed withlight, the light beam reflected by each micro mirror is incident on theprojector system and converted thereby to imaging light. The imaginglight forms an enlarged image of the image on the light bulb Lb on thetarget surface. This image is called as projected image.

Since the light bulb is illuminated with light with uniform luminancedistribution, the projected image has uniform illumination distribution.Thus, a digital image is displayed on the target surface.

The projector functions to project an image as a real image of the imagedisplayed on the light bulb LB onto the target surface such as a screen.The size of the projected image or the distance from the projector tothe target surface differs depending on the specific condition of theprojector in use.

A projected image needs to be brought into focus on the target surface.FIG. 2A shows a projector system POS1 including lenses in coaxial,rotational symmetry with an optical axis AX. The focus of an image onthe screen SC is generally adjusted by moving the entire projectorsystem or moving a focus lens group.

FIG. 2B shows a projector which uses a projector system comprising arefracting optical train POSL1 and a mirror train POSM1 (a singleconcave mirror in the drawing) not coaxial with the refracting opticaltrain POSL1. It aims to project images in a closer distance than arelated art projector.

To be easily viewable in an extremely close distance, an image needs tobe projected above the projector. The light bulb LM (DMD) iseccentrically disposed with its center off the optical axis AX of theprojector system as shown in FIG. 2A, 2B. To realize a wide projectionarea and maintain image quality, a wide angle lens is used for theprojector system. However, there is a limitation to widening the angleof the projector system of lenses in coaxial, rotational symmetry, andan optical path has to be extended by a mirror train in order to projectimages from an extremely close position adjacent to the screen SC.

An image can be projected in a close distance by oblique projection bywhich the optical path of the projector system is reflected by a planarmirror to incline the optical axis thereof relative to the screen.However, this type of projection faces a problem that a projected imageis distorted to a trapezoidal shape.

In FIG. 2B the concave mirror of the mirror train POSM1 with a free-formcurved surface can effectively correct a trapezoidal distortion in aprojected image. The correction of trapezoidal distortion with afree-form curved mirror is disclosed in detail in “Optical andElectro-optical Engineering Contact, Vol. 39, No. 9 in 2001 by JapanOptomechatronics Association”.

A floating focus system is suitable for the projector comprising therefracting optical train POSL1 and the mirror train POSM1 including afree-form curved surface to correct a trapezoidal distortion in FIG. 2B.This system is to perform focusing at an extremely close range by fixingone or more lenses closest to the light bulb LB and moving the otherlens groups and mirrors along the optical axis like moving “floatingtrees”. It is widely applied for an interchangeable lens of a singlelens reflex camera.

However, it is not possible to sufficiently correct the trapezoidaldistortion in an image projected at an extremely close distance byfocusing with a single lens or lens groups or protruding the entireprojector system. Further, curvature of field cannot be sufficientlycorrected, leading to blurs in the center and periphery of the display.

Meanwhile, the floating focus system can properly correct trapezoidaldistortion and curvature of field in an image projected from anextremely close distance by the non-coaxial curved mirror.

This is described in detail referring to FIGS. 3A, 3B. FIG. 3A shows aprojector which obliquely projects images. A projector system POS0projects an image on the screen SC. A planar mirror to reflect anoptical path is omitted therefrom for simplicity.

The display surface of the light bulb LB as DMD is of a rectangularshape with vertical (Y direction) short sides but the projected image isa trapezoidal shape as shown in FIG. 3B.

FIG. 4A shows another projector comprising a projector system having arefracting optical train POSL1 and a mirror train POSM1 as a concavemirror. The surface of the light bulb LB on which images are displayedis rectangular as shown in FIG. 4B. The refracting optical train POSL1forms a real image on the display as an intermediate image Im0 betweenthe refracting optical train POSL1 and the mirror train POSM1. Themirror train POSM1 projects the intermediate image Im0 on the screen SCas an object image.

The intermediate image Im0 formed by the refracting optical train POSL1is distorted to a trapezoid with a narrow top portion as shown in FIG.4B. The distortion is corrected by the mirror train POSM1 and acorrected rectangular image is projected on the screen SC, as shown inFIG. 4B.

To project a smaller image onto the screen SC with the projector in FIG.4A, the screen SC is moved rightward in Z direction from the position inFIG. 4A and focus adjustment is performed by protruding the coaxialrefracting optical train POSL1 in Z direction as shown in FIG. 5A.

The distortion in the intermediate image Im0 shows almost no changebefore and after the protrusion of the refracting optical train POSL1and the shape thereof in FIG. 5B is similar to that of the screen inFIG. 4B. Accordingly, a trapezoidal distorted image in FIG. 5B isprojected on the screen SC.

This effect is described in detail with reference to FIGS. 6A to 6D.FIG. 6A shows an X to Z cross section of FIG. 4A. In FIG. 6A theprojector system including the concave mirror POSM1 projects light atdifferent angles upward and downward in Y direction on an XZ crosssection of the screen SC. FIG. 6C shows a rectangular image properlyprojected by the projector system in FIG. 6A.

When the screen SC is moved as in FIG. 5A, a trapezoidal distortion witha short top side occurs due to light's reaching different positions in Xdirection on top and bottom of the screen SC, as shown in FIG. 6B.

When the light bulb and the refractive optical system are disposed in aproper distance along the normal line of the light bulb, the floatingfocusing is very effective to correct trapezoidal distortion in an imageand curvature of field. Further, owing to the good correction ofcurvature of field, the floating focusing is effective when focusadjustment amounts are largely different in the top and bottom of thedisplay, for example, when the screen SC is moved to the curved mirrorPOSM1 from a position SC1 (FIG. 4A) to a position SC2 (FIG. 5A).

Meanwhile, for correcting the same focus adjustment amount on the entirescreen, not the floating focusing but the focusing by protruding theentire projector system or the front lens group is effective.

Various methods for the focusing of the projector are well known, forexample, disclosed in Japanese Patent Application Publication No.2009-251457, No. 2009-229738, and No. 2008-165187.

Thus, floating focusing can correct blurs in the center and periphery ofan image on the display but it cannot deal with blurs in the entireimage due to a variation in the distance between the refractive opticalsystem and the light bulb or a variation in the focal length of therefractive optical system.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a projector type imagedisplay device comprising a projector system made up of a refractiveoptical system and a mirror train and able to correct blurs in thecenter and periphery of a display and the entire projected image.

According to one aspect of the present invention, an image displaydevice comprises an image display element, a light source, anillumination system, a projector system comprising a refractive opticalsystem including a plurality of lens groups, and a mirror trainincluding a curved mirror, a first focus structure configured to movethe respective lens groups of the refractive optical system by differentamounts along a normal line of the image display element, and a secondfocus structure configured to move the respective lens groups along thenormal line of the image display element by different amounts from thoseof the first focus structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, embodiments, and advantages of the present invention willbecome apparent from the following detailed description with referenceto the accompanying drawings:

FIG. 1 shows the structure of a general projector;

FIGS. 2A, 2B show the focus adjustment of a projected image;

FIGS. 3A, 3B show a trapezoidal distortion of a projected image;

FIGS. 4A, 4B show the correction of the trapezoidal distortion;

FIGS. 5A, 5B show an example of insufficient correction of thetrapezoidal distortion of an image projector using a mirror train;

FIGS. 6A to 6D show another example of insufficient correction of thetrapezoidal distortion;

FIG. 7 shows the image projector using a mirror train when the positionof a screen is changed;

FIG. 8 shows the essential part of an image display device according toa first embodiment;

FIG. 9 shows an image display device according to a second embodiment;

FIG. 10 shows an image display device according to a third embodiment;

FIGS. 11A, 11B show examples of a refractive optical system used in thedevices according to the first to third embodiments;

FIG. 12 shows an example of the structure of the refractive opticalsystem;

FIG. 13 is a table showing data on the refractive optical system in thefirst embodiment;

FIGS. 14A to 14C show specific lens data in the first embodiment;

FIGS. 15A, 15B show data on the surface shapes of concave mirrors in thefirst embodiment;

FIG. 16 shows an example of a projector system mounted in an exteriorpackage OC; and

FIG. 17 shows another example of a projector system mounted in theexterior package OC.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Wherever possible,the same reference numbers will be used throughout the drawings to referto the same or like parts.

The following embodiments will describe a DMD as a light bulb by way ofexample. However, an image display element should not be limited to theDMD and various light bulbs such as LD panel, LCOS panel can be used.

Note that in the drawings a light source and an illumination system toguide light to a light bulb from a light source are omitted forsimplicity. In reality the illumination system in FIG. 1 comprising theintegrator rod IR, lens LN, mirror M, and curved mirror to illuminatethe light bulb LB is used. Needless to say that the type of a lightsource or an illumination system can be arbitrarily determined inaccordance with the type or configuration of an image display element.

First Embodiment

FIG. 8 shows the essential part of an image display device according toa first embodiment. The image display device comprises an illuminationsystem, a projector system including a refractive optical system, alight source and a light bulb LB. The light bulb LB is a digital micromirror device (DMD) and held in a housing HS. A lens barrel CL isconnected with the housing HS via a first intervenient element InA. Therefractive optical system is contained in the lens barrel CL and itcomprises first to fourth lens groups LI, LII, LIII, LIV arranged inthis order from the light bulb side.

The image display device also comprises a reflecting mirror RM held in aholder HL via a second intervenient element InB, and a concave mirrorCNM as a mirror train.

A light beam from the light source illuminates the display surface ofthe light bulb LB via the illumination system, and is reflected thereby,changed in intensity by an image displayed on the surface and incidenton the refractive optical system.

Then, the light beam is reflected by the reflecting mirror RM andconcave mirror CNM to a not-shown screen as a target surface to form, onthe screen, an image enlarged from the image displayed on the lightbulb.

It is preferable for the optical system to converge the light beam oncevia the reflecting mirror RM by forming a real image on the light bulbas an intermediate image on an optical path between the refractiveoptical system and the concave mirror CAN, for the purpose of reducing adispersion of the reflected light by the reflecting mirror RM.

Now, focus adjustment by floating focusing is described. Although notshown in FIG. 8, when the position of the screen is changed from theposition SC1 to SC2 in FIG. 7, different focus adjustment amounts on thetop and bottom of the screen are corrected by floating focusing. In thepresent embodiment the second and third lens groups LII, LIII of therefractive optical system are moved along the optical axis.

The lens barrel CL includes not-shown cam grooves inside, for example,and pins of the second and third lens groups LII, LIII are fitted intothe cam grooves. Thereby, the second and third lens groups LII, LIII canbe moved in different directions by rotating the lens barrel CL. Thiscam mechanism is a first focus structure. Although not shown, the lensbarrel CL is of a complex structure comprising several elements.

Meanwhile, the size of manufactured housings varies so that a distancebetween the light bulb LB and the first lens group LI also varies in thelens barrel CL mounted in the housing in accordance with a variation inthe size of the housing HS. Similarly, manufactured lenses and concavemirrors used in the refractive optical system also vary in shape,causing a variation in the focal length of the refractive optical systemand in the optimal distance between the light bulb LB and the first lensgroup LI.

To properly set the distance between the light bulb LB and the firstlens group LI even with the two kinds of variation, the image displaydevice according to the present embodiment comprises a firstintervenient element InA between the light bulb LB and the first lensgroup LI.

The first intervenient element InA can be made of thin plates inthickness of about 0.1 mm and the total thickness thereof is adjustableby changing the number of the thin plates. Accordingly, the distancebetween the light bulb LB and the first lens group LI is adjustable inunit of 0.1 mm.

Alternatively, the first intervenient element InA can be aluminum platesin different thicknesses in unit of 0.1 mm, for example. One of theplates in an optimal thickness for a combination of the housing HS andthe refractive optical system is selected and inserted therebetween.Thereby, the distance between the light bulb LB and the first lens groupLI is adjustable in unit of 0.1 mm.

It is preferable to fix the first intervenient element InA, housing HS,and lens barrel CL with screws, for example after adjusting theirpositions.

Further, the image display device comprises a second intervenientelement InB between the reflecting mirror RM and holder HL, to adjust anoptical path length between the fourth lens group LIV and concave mirrorCNM and correct a displacement of the positions thereof from theassembled positions. The displacement occurs because of the adjustmentof the positions of the light bulb LB and the first lens group LI.

The second intervenient element InB can be aluminum plates in differentthicknesses or made of thin plates as the first intervenient elementInA, to be able to easily, precisely adjust the optical path length.

Further, with use of thin plates for the first and second intervenientelements InA, InB, the first intervenient element InA and housing HS orthe second intervenient element InB and holder HL can be fixed at pluralpoints with screws, for example. By changing the number of the platesinserted therebetween at each fixing point, it is possible to correct anerror in the inclination of the light bulb LB and refractive opticalsystem or the curved mirror CNM and refractive optical system.

The first and second intervenient element InA, InB constitute a secondfocus structure.

According to the present embodiment, it is made possible to adjust thedistance between the lens barrel CL and the housing HS by simplyinserting the first intervenient element InA of a simple structure and aspecial structure is unnecessary. Since the housing HS which is close tothe light bulb LB and receives heat therefrom does not directly contactthe lens barrel CL, the heat is not easily transmitted to the lensbarrel CL. Thus, out-of-focus images due to thermal expansion of lensescan be prevented.

As described above, the image display device according to the firstembodiment comprises the first and second focus structures. The firstfocus structure is configured to perform focus adjustment of a projectedimage by floating focusing in which the second and third lens groupsLII, LIII of the refractive optical system are moved by differentamounts along the normal line of the light bulb, when the distancebetween the screen and concave mirror CNM is changed.

The second focus structure is configured to properly position the lightbulb, refractive optical system, and concave mirror (mirror train) inthe assembly of the optical systems of the image display device andbring a projected image into focus at default position.

Second Embodiment

Now, an image display device according to a second embodiment isdescribed with reference to FIG. 9. The second embodiment differs fromthe first embodiment in that a screw structure BS is provided to adjustthe distance between the light bulb LB and first lens group LI inreplace of the first intervenient element InA.

The screw structure BS adjusts the distance between the light bulb LBand first lens group LI. It constitutes the second focus structuretogether with the second intervenient element InB, so that the distanceis adjusted with the screw structure in the assembly process of theoptical systems and the screw structure BS is secured after theadjustment, so as not to allow a user to adjust it.

Note that the second focus structure should not be limited to the firstand second intervenient elements InA, InB and the screw structure BS.Any structure can be arbitrarily used as long as it can achieve the samefunctions.

Third Embodiment

An image display device according to a third embodiment is describedwith reference to FIG. 10. In the present embodiment the refractiveoptical system is comprised of the first to fourth lens groups LI toLIV.

In the present embodiment the focus adjustment of a projected image isdone by floating focusing in which the second and third lens groups LII,LIII are moved. The fourth lens group LIV is fixed. That is, the secondand third lens groups LII, LIII and a moving mechanism thereforconstitute the first focus structure.

Meanwhile, the second focus structure is comprised of the fourth lensgroup LIV and a structure to move it along the optical axis.

Moving the fourth lens group LIV along the optical axis can attain thesame focusing effects as those by the protrusion of the entire lensbarrel in the first and second embodiments. The fourth lens group LIV ismoved by a cam structure different from the one for the second and thirdlens groups LII, LIII in the assembly process of the optical systems.After the adjustment, it is fixed so as not to allow a user to operateit.

While an image is projected on a target surface, the first focusstructure moves the second and third lens groups LII, LIII for the focusadjustment of the image. Moving the fourth lens group IV is morepreferable than the focusing by the protrusion of the entire lens barrelsince the first and second focus structures are independent cammechanisms and a unit of lens moving amount for focusing is small sothat an error such as inclination caused from the adjustment is reduced.

However, with use of the reflecting mirror RM in FIGS. 8, 9, in movingthe fourth lens group LIV away from the light bulb LB as shown in FIG.10, it is necessary to dispose the fourth lens group IV so as not toblock the light beam reflected by the reflecting mirror RM to theconcave mirror CNM.

The examples of the structure of the refractive optical system accordingto the first to third embodiments are described with reference to FIGS.11 to 17.

In FIGS. 11A, 11B an illumination system includes a lamp 1 as a lightsource, an integrator rod 2, a lens 3, a mirror 4, a curved mirror 5,and a concave mirror with a free-form curved surface (equivalent to theconcave mirror CNM). The curved mirror 5 is a concave mirror with aspherical reflective surface. FIG. 11B shows a refractive optical system81.

A projector system 8 includes a light bulb 7, and a protective coverglass 6 for the light bulb, a protective glass 9 for the concave mirror,a lens barrel 10 containing the first to fourth lens groups.

The lens barrel 10 includes three different cam grooves to move three ofthe four lens groups separately. Note that the cam groove closest to thelight bulb 7 does not move at all so that it is irrelevant of focusing.

FIG. 12 shows an example of the structure of the refractive opticalsystem 81. It is made up of four lens groups of 11 lenses. A first lensgroup LI is made of 6 lenses, a second lens group LII is a single lens,a third lens group LIII is made of 3 lenses and a fourth lens group LIVis a single lens.

Specific data on the lenses of the refractive optical system in FIG. 12are shown in FIGS. 13 to 15. FIG. 13 shows the curvature radius of eachlens surface, face interval (mm), the refractive index and Abbe numberof material, the aperture radius of a diaphragm, eccentricity Y and α,and aspheric surfaces.

Curvature radius at 0.000 represents infinity ∞, that is, planarsurface. Eccentricity Y is a shift amount (mm) of the refractive opticalsystem 81 along the optical axis downward in Y (vertical) direction(FIG. 5A). Eccentricity α is a shift amount (mm) of the concave mirrorand protective glass relative to a plane including the optical axis (Zdirection) and a lateral direction of the light bulb. The asterisk * inthe aspheric surface represents that the lens surface is aspherical.

LB (0) in the face No. column represents a display surface of the lightbulb and No. 1, 2 are both surfaces of a cover glass.

Curvature radius “1.0E+18” represents “1*10¹⁸” and the surfaces withthis curvature radius is approximately planar. The values of theaspheric surface are paraxial curvature radius.

FIG. 14A shows data on the aspheric surfaces in FIG. 13. The asphericsurfaces can be expressed by a known formula:

D=CH ²/[1+√{1−(1+K)}C ² H ² ]+ΣE ₂ jH ^(2j)(j=1 to 8)

where C is paraxial curvature (inverse of paraxial curvature radius), Kis conic constant, E2j is high-order aspheric coefficient (J=2 to 8), His coordinate in orthogonal direction to the optical axis, and D isdepth along the optical axis.

In the fourth embodiment the conic constants of the aspherical surfacesof the refractive optical system are all zero.

FIG. 14B shows the position data of the reflecting mirror, concavemirror, first and second surfaces of the protective glass, and first andsecond screen distances (SC1, SC2 in FIG. 7) in X, Y, Z directionsrelative to the vertex of the lens surface closest to the reflectingmirror.

FIG. 14C shows specific moving amounts (mm) of the lens groups at thefirst and second screen distances. FIGS. 15A, 15B show data on thesurface shape of the concave mirror CNM.

The surface of the concave mirror CNM is a free-form curved surface, andit is expressed by the following formula:

Z=cr ²/[1+√{1−(1+k)c ² r ² }]+ΣC _(j) *x ^(m) y ^(n)

where c is paraxial curvature radius, K is conic constant, C_(j) is ahigh-order coefficient (j=2 to 72), r is a distance in orthogonaldirection relative to the optical axis, z is a sag amount of a surfaceparallel to the optical axis, and x, y are coordinates in X and Ydirections in FIGS. 5A, 5B, respectively.

In FIGS. 15A, 15B “x**4*y**7” in C40 represents x⁴xy⁷, for example.

The first lens group LI is made of six lenses and fixed during the useof the image display device. The second and third lens groups LII, LIIIare moved by different amounts for floating focusing. Further, at theassembly of the optical systems, the focus of an entire projected imageis adjusted by moving the fourth lens group LIV and thereafter, thefourth lens group LIV is fixed.

FIGS. 16, 17 show examples of a projector with a projector systemmounted in an exterior package OC. The light bulb LB, refractive opticalsystem POSL, and mirror trains RM, CNM are contained in the exteriorpackage OC. A light beam is projected therefrom via a cover glass CG andimaged on the screen SC.

The lens barrel is mechanically structured so that a focus lever of thefirst focus structure for floating focusing is exposed outside theexterior package OC. For example, if the screen SC is moved verticallyor in Y direction in FIG. 16, the image can be brought into focus bymoving the focus lever. Thus, images with good resolution as a whole canbe projected on the screen.

The second focus structure is accommodated inside the exterior packageOC and cannot be exposed outside, so as not to allow a user to operateit.

With no use of the reflecting mirror, the second focus structure can bea protrusion mechanism to move the entire refractive optical systemalong the normal line of the light bulb. It can be a lens barrel with ascrew mechanism to hold the refractive optical system and be protrudedby the rotation of the screw mechanism.

Alternatively, the protrusion mechanism can be comprised of a lensbarrel holding the refractive optical system and one or moreintervenient elements provided between the lens barrel and the housingcontaining the light bulb. The housing and the lens barrel can beintegrated via the intervenient elements.

Further, the second focus structure can be a front focus mechanism tomove one lens group of the refractive optical system farthest from thelight bulb along the normal line of the light bulb.

In the image display device the refractive optical system and the mirrortrain are positioned so that a light beam from the light bulb isincident on the mirror train via the refractive optical system andreflected thereby to the target surface. The projector system caninclude, between the refractive optical system and the mirror train, areflecting mirror to bend an optical path and be held in a structurewhich is adjustable of the position of the reflecting mirror.

The structure holding the reflecting mirror can be configured of aholder for the reflecting mirror and one or more intervenient elementsdisposed between the holder and the reflecting mirror. The curved mirrorof the mirror train is a concave mirror. A real image on the light bulbcan be formed as an intermediate image on the optical path between therefractive optical system and the mirror train.

The first focus structure moves the lens groups of the refractiveoptical system along the normal line of the light bulb by differentamounts for floating focusing of a projected image on the screen.

Moreover, the second focus structure moves the lens groups of therefractive optical system along the normal line of the light bulb bydifferent amounts from those of the first focus structure. Thereby, itis possible to effectively correct blurs in the entire projected imagedue to a variation in the distance between the refractive optical systemand the light bulb and a variation in the focal length of the refractiveoptical system.

The second focus structure cannot be operated by a user after theprojector is assembled into the exterior package. Thus, the two focusstructures do not confuse the user when he operates the projector.

Although the present invention has been described in terms of exemplaryembodiments, it is not limited thereto. It should be appreciated thatvariations or modifications may be made in the embodiments described bypersons skilled in the art without departing from the scope of thepresent invention as defined by the following claims.

What is claimed is:
 1. An image display device comprising: an image display element; a light source; an illumination system; a projector system comprising a refractive optical system including a plurality of lens groups, and a mirror train including a curved mirror; a first focus structure configured to move the respective lens groups of the refractive optical system by different amounts along a normal line of the image display element; and a second focus structure configured to move the respective lens groups along the normal line of the image display element by different amounts from those of the first focus structure.
 2. An image display device according to claim 1, wherein the second focus structure is a protrusion mechanism to move the entire refractive optical system along the normal line of the image display element.
 3. An image display device according to claim 2, wherein the protrusion mechanism is comprised of a lens barrel to hold the refractive optical system and a screw mechanism and protrudes by a rotation of the screw mechanism.
 4. An image display device according to claim 2, wherein: the protrusion mechanism is comprised of a lens barrel to hold the refractive optical system and one or more intervenient elements provided between the lens barrel and a housing to hold the image display element; and the lens barrel and the housing are integrated with each other via the intervenient elements.
 5. An image display device according to claim 1, wherein the second focus structure is a front focus mechanism to move one of the lens groups of the refractive optical system as farthest from the image display element along the normal line of the image display element.
 6. An image display device according to claim 1, wherein the second focus structure is fixed during operation of the first focus structure.
 7. An image display device according to claim 1, wherein: the refractive optical system and the mirror train are disposed so that a light beam is incident on the mirror train from the image display element via the refractive optical system and reflected by the mirror train to a target surface to project an image on the target surface; the projector system includes a reflecting mirror between the refractive optical system and the mirror train to bend an optical path; and the reflecting mirror is held in a structure which is able to adjust a set position of the reflecting mirror.
 8. An image display device according to claim 7, wherein the structure is comprised of a holder for the reflecting mirror and one or more intervenient elements provided between the holder and the intervenient elements.
 9. An image display device according to claim 7, wherein the curved mirror of the mirror train is a concave mirror to form a real image on the image display element as an intermediate image on an optical path between the refractive optical system and the mirror train. 